While not limited thereto, the present invention is particularly adapted to the use of hydraulic power for the airless spraying of viscous fluid or semifluid products such as grease, oil, paint, gel-like compounds, sealants, undercoating materials, and the like. More particularly, the present invention relates to a method and apparatus to control a hydraulically-powered pump used to pressurize such materials over an infinitely adjustable range of pressures that includes relatively high pressure, i.e., of the order of 10,000 pounds per square inch, for discharge of the pressurized material from a nonclosable or synchronously opened orifice in the casing of a spray device.
In the past, the nozzle opening of an airless spray gun has been supplied with the material to be sprayed at pressures ranging from 800 to 900 pounds per square inch. It is known in the art to pressurize paint, for example, to pressures of about 2,000 pounds per square inch by utilizing a positive displacement pump. The pressurized material is discharged by controllably opening a normally closed orifice of an airless spray gun. So far as I am aware, materials have not been delivered to an airless spray gun under a developed pressure of the order of 10,000 pounds per square inch and even higher pressures.
As is known, grease lubricators, oil transfer pumps and spray systems for low viscosity materials have been powered by air motors or piston and cylinder assemblies that are coupled to a controlled source of pneumatic pressure. Electric solenoids and electric motors have also been used to provide the required driving force for pumps to pressurize material of the foregoing types. When it is desired to deliver such materials to an airless spray gun at pressures of the order of 10,000 pounds per square inch in an environment that requires portability of the spray system, it is not economically feasible or practical to employ electrical generating equipment to develop the necessary electric power for a motor or solenoid to develop the forces necessary to pressurize the material to such a magnitude. The use of a rotary type pneumatic motor or pneumatically-powered piston and cylinder assembly demands not only a large capacity air supply system, but also there are certain acute and inherent disadvantages to the use of pneumatics as a motivating force for a pump. For example, high volumes of air at even relatively moderate pressures create severe moisture problems in a pneumatic supply system, particularly in an outside environment where temperatures may fall below freezing and even below 0.degree. F. The storage tank, control devices and supply lines of a pneumatic system exposes workmen to severe hazards. Moreover, the required horsepower input to a portable pneumatic system is excessively large and the equipment is cumbersome to manipulate and control. Control of the power supply system for high pressure spraying of materials becomes critically important when such materials are delivered to an airless spray gun at pressures up to the order of 10,000 pounds per square inch and even greater pressures.
In known lubricator or spray systems, the orifice of the discharge apparatus is simply closed through the use of a control lever to terminate the discharge of pressurized material. This causes the pump to proceed to a stall condition which occurs when a preset maximum pressurization of the material is exceeded. However, if the discharge orifice of an airless spray gun were blocked to terminate the discharge of materials pressurized to 10,000 pounds per square inch, the failure of a valve or other control means to act quickly will result in a momentary or even a longer period of time during which the material is pressurized to dangerously high pressures. Such abnormally high pressures would quickly reach 30,000 pounds per square inch or higher. Should this occur, material supply lines may burst and workmen would obviously be subject to an extremely hazardous working condition. Thus, I have done away with the conventional concepts for controlling the discharge of material in a lubricator or spray discharge system including the usual procedure of closing the discharge orifice of the airless spray gun to thereby cause the pumping system to proceed to a stall condition.